Field-induced π-polarization in barrelene derivatives: a computational study based on structural variation

Abstract

Barrelene, H–C(CH=CH)₃C–H, is an unsaturated polycyclic hydrocarbon containing three isolated double bonds in a non-planar arrangement. We have studied the transmission of field effects through the barrelene framework by analyzing the small structural changes occurring in the phenyl group of many Ph–C(CH=CH)₃C–X molecules, where X is a variable substituent. Molecular geometries have been determined by quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Comparison with the results obtained for the corresponding saturated molecules, the bicyclo[2.2.2]octane derivatives Ph–C(CH₂–CH₂)₃C–X, reveals a small, but significant, field-induced π-polarization of the barrelene cage, especially when the remote substituent is a charged group. Additional evidence of π-polarization is obtained by comparing the electric dipole moments of the two sets of uncharged molecules. The structural variation of the barrelene cage caused by the variable substituent in Ph–C(CH=CH)₃C–X molecules has also been investigated. It is much larger than that of the phenyl group and depends primarily on the electronegativity of the substituent. Particularly pronounced is the concerted variation of the non-bonded distance between the bridgehead carbons of the cage, r(C···C) ^BARR₁ , and the average of the three C–C–C angles at the cage carbon bonded to the variable substituent, α ^BARR₁ . A scattergram of r(C···C) ^BARR₁ versus the corresponding parameter for bicyclo[2.2.2]octane derivatives, r(C···C) ^BCO₁ , shows that the variation of r(C···C) ^BARR₁ becomes gradually less pronounced than that of r(C···C) ^BCO₁ as the electronegativity of the substituent increases.